Power converter with snubber

ABSTRACT

A buck or boost converter, which includes a first inductor, a controlled switch, a main diode, and an output capacitor, also includes a snubber circuit to reduce losses. The snubber circuit includes a second inductor in a path in series with the switch and main diode of the converter, a series-connected resistor and diode connected directly in parallel with the second inductor, and a capacitance in parallel with the main diode and which can be constituted partly or entirely by parasitic capacitance of the main diode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/652,613, entitled “POWER CONVERTER WITH SNUBBER”, and filed on Jan.12, 2007, the entire contents of which are incorporated herein byreference.

FIELD OF INVENTION

This invention relates to power converters, and is particularlyconcerned with a power converter in which losses are reduced by using asnubber circuit without requiring an additional switch.

BACKGROUND

In a conventional boost converter, an input voltage is coupled via aninductor to a switch, typically a MOSFET, and a diode and a capacitor inseries are coupled in parallel with the switch, an output voltage of theconverter being derived from the capacitor. In the absence of atransformer, the output voltage is greater than the input voltage. Theswitch is alternately opened and closed, typically at a high frequencyand with a controlled duty cycle.

An increasingly important application of boost converters is for powerfactor correction (PFC) in so-called offline power supply arrangementsfor consumer electronics equipment. In such arrangements typically arectified AC power supply is converted by a boost converter to a highoutput voltage to provide a near-unity power factor; the output voltagecan be used directly or converted by one or more other power convertersto one or more AC and/or DC voltages for use.

Operation of a boost converter in discontinuous current mode (DCM), inwhich the converter switch is turned on when the inductor current iszero, has the results that the peak current is twice the average currentand the inductor current has large swings, requiring a relatively largecore involving increased losses. With increasing converter power levels,for example for power levels greater than about 200 or 300 W as may berequired for a boost converter for PFC, it is preferable to operate theboost converter in continuous conduction mode (CCM), in which theconverter switch is turned on before the inductor current has fallen tozero. A boost converter operated in CCM has relatively smaller inductorcurrent swings and peak current.

In consequence, the diode of the boost converter, referred to as theboost diode, is required to have a very fast reverse recovery,especially in view of the typical high output voltage of a boostconverter used for PFC. For example, such a boost converter maytypically be desired to operate with a peak input voltage up to about360V, and the output voltage may conveniently be selected to be about380 to 400V. During the reverse recovery period, immediately after theconverter switch is turned on so that the diode is reverse biased, afterhaving been forward biased and conducting the non-zero inductor current,the diode is still conductive due to carriers in the diode junctionregion, and very large reverse currents can flow, substantiallyincreasing the stress and power loss in the converter switch.

The diode of a boost converter used for PFC can be based on siliconcarbide semiconductor technology, but such diodes may have a cost of theorder of ten times that of silicon diodes. Even with a diode that doesnot exhibit reverse recovery behaviour, the converter switch is turnedon and off with the full current of the inductor flowing, resulting insubstantial switching losses.

In order to reduce these disadvantages, it is known to provide morecomplex arrangements of a boost converter incorporating an additional orauxiliary switch. Examples of such converters are described in Bassettet al. U.S. Pat. No. 5,446,366 issued Aug. 29, 1995 and entitled “BoostConverter Power Supply With Reduced Losses, Control Circuit And MethodTherefor”; Jovanovic U.S. Pat. No. 5,736,842 issued Apr. 7, 1998 andentitled “Technique For Reducing Rectifier Reverse-Recovery-RelatedLosses In High-Voltage High Power Converters”, and in Jang et al. U.S.Pat. No. 6,051,961 issued Apr. 18, 2000 and entitled “Soft-SwitchingCell For Reducing Switching Losses In Pulse-Width-Modulated Converters”.

The additional complexities and additional switch of such knownconverters add to their cost, as well as to the complexity and cost ofthe control circuit which must be provided for controlling the switchesof the boost converters.

It is also known from Farrington et al. U.S. Pat. No. 5,550,458 issuedAug. 27, 1996 and entitled “Low-Loss Snubber For A Power FactorCorrected Boost Converter” to provide a boost converter with a snubberto reduce diode reverse recovery and switching losses without providingthe converter with an additional switch. In this converter a snubberinductor is connected in series with the boost diode, and a resistor inseries with a snubber diode is connected in parallel with theseries-connected boost diode and snubber inductor. This arrangement hasthe disadvantage of requiring a further diode connected to the junctionbetween the boost diode and the snubber inductor to prevent ringing ofthe voltage across the boost diode when the switch is on, with aresulting current circulating through the snubber inductor, this furtherdiode, and the converter switch. This reference also discloses a similarsnubber arrangement applied to a buck converter.

Another boost converter with a snubber circuit, having the disadvantageof further complexity, is known from Kim U.S. Pat. No. 5,633,579 issuedMay 27, 1997 and entitled “Boost Converter Using An Energy ReproducingSnubber Circuit”.

There remains a need to provide a power converter, such as a boostconverter or a buck converter, with reduced switching and/or reverserecovery losses using a relatively simple arrangement without anadditional switch.

SUMMARY

According to an aspect of the invention, a power converter comprises:three terminals; a switch controlled by a control signal; a first diodecoupled to the switch in a path between two of the three terminals; afirst inductor coupling a junction between the switch and the firstdiode to the other of the three terminals; a circuit coupled in a pathwith one of the switch and the first diode, the circuit comprising asecond inductor, a second diode, and a resistor coupled to the seconddiode, the resistor and the second diode being coupled in a path acrossthe second inductor; and a capacitor coupled across the first diode, oracross a path in which the first diode is coupled to the second diode,or across a path in which the first diode is coupled to the resistor.

The three terminals comprise an input terminal, a voltage referenceterminal, and an output terminal in one embodiment.

In a boost configuration, the path between two of the three terminalscomprises a path between the voltage reference terminal and the outputterminal, and the first inductor is coupled to the input terminal. Thecircuit and the first diode might be coupled in a path between theswitch and the output terminal. The circuit and the switch might becoupled in a path between the voltage reference terminal and the firstdiode.

In a buck configuration, the path between two of the three terminalscomprises a path between the input terminal and the voltage referenceterminal, and the first inductor is coupled to the output terminal. Thecircuit and the switch might be coupled in a path between the inputterminal and the first diode. The circuit and the first diode might becoupled in a path between the switch and the voltage reference terminal.

Another aspect of the invention provides a boost converter comprising: avoltage reference terminal; an input terminal to receive an inputvoltage relative to the voltage reference terminal; an output terminalto provide an output voltage relative to the voltage reference terminal;a switch controlled by a control signal; a first diode coupled to theswitch in a path between the output terminal and the voltage referenceterminal; a first inductor coupling a junction between the first andsecond switches to the input terminal; a circuit coupled in a path withone of the switch and the first diode, the circuit comprising a secondinductor, a second diode, and a resistor coupled to the second diode,the resistor and the second diode being coupled in a path across thesecond inductor; and a capacitor coupled across the first diode, oracross a path in which the first diode is coupled to the second diode,or across a path in which the first diode is coupled to the resistor.

In some embodiments, the circuit and the first diode are coupled in apath between the output terminal and the switch.

The circuit and the switch could be coupled in a path between thevoltage reference terminal and the first diode.

Where the capacitor is coupled across the first diode, the convertermight also include a further capacitor coupled across the resistor.

A buck converter is also provided, and comprises: a voltage referenceterminal; an input terminal to receive an input voltage relative to thevoltage reference terminal; an output terminal to provide an outputvoltage relative to the voltage reference terminal; a first diode; acontrolled switch coupled to the first diode in a path between the inputterminal and the voltage reference terminal; a first inductor coupling ajunction between the switch and the first diode to the output terminal;a circuit coupled in a path with one of the switch and the first diode,the circuit comprising a second inductor, a second diode, and a resistorcoupled to the second diode, the resistor and the second diode beingcoupled in a path across the second inductor; and a capacitor coupledacross the first diode, or across a path in which the first diode iscoupled to the second diode, or across a path in which the first diodeis coupled to the resistor.

In some embodiments, the switch and the circuit are coupled in a pathbetween the input terminal and the first diode.

The first diode and the circuit could be coupled in a path between theswitch and the voltage reference terminal.

Where the capacitor is coupled across the first diode, the convertermight also include a further capacitor coupled across the resistor.

These converters may also include an output capacitor coupled across theoutput terminal and the voltage reference terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be further understood from thefollowing description by way of example with reference to theaccompanying drawings, in which the same references are used indifferent figures to represent corresponding elements and in which:

FIG. 1 schematically illustrates a known boost converter having asnubber circuit;

FIG. 2 schematically illustrates a boost converter in accordance with anembodiment of this invention;

FIG. 3 schematically illustrates a modified form of the boost converterof FIG. 2 in accordance with another embodiment of this invention;

FIG. 4 illustrates simplified waveforms of voltages and currents thatcan occur in operation of the boost converter of FIG. 2;

FIG. 5 illustrates the simplified waveforms of FIG. 4 on an expandedtime scale, around a switch turn-on time of the boost converter;

FIG. 6 illustrates the simplified waveforms of FIG. 4 on an expandedtime scale, around a switch turn-off time of the boost converter;

FIG. 7 schematically illustrates a buck converter in accordance with anembodiment of this invention;

FIG. 8 schematically illustrates another buck converter in accordancewith a further embodiment of this invention;

FIG. 9 schematically illustrates another boost converter in accordancewith an embodiment of the invention; and

FIGS. 10 to 12 illustrate modifications of the boost converter of FIG. 2in accordance with further embodiments of the invention.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 schematically illustrates a boostconverter having a snubber circuit, which is known from U.S. Pat. No.5,550,458 referred to above. The boost converter itself comprises aninductor 10, a switch 12, a diode 14, and a capacitor 16. A positiveinput voltage Vin from a suitable source (not shown), relative to a zerovolt (0V) line is coupled via the inductor 10, referred to as a boostinductor, to the switch 12 which typically can be, and in FIG. 1 isshown as being, constituted by a MOSFET with its drain connected to theinductor 10, its source connected to the 0V line, and a gate to which apulsed control signal G is applied in known manner for controlling thestate of the switch 12. FIG. 1 also shows a so-called body diodeinherent to the MOSFET, having its anode connected to the source and itscathode connected to the drain of the MOSFET.

The junction between the drain of the MOSFET switch 12 and the inductor10 is coupled, in the case of FIG. 1 via an inductor 20 which forms partof the snubber circuit, to the anode of the diode 14, referred to as aboost diode or rectifier. The cathode of the diode 14 is connected to anoutput terminal of the converter for a positive output voltage Vout,relative to the 0V line, and to one terminal of the capacitor 16,referred to as an output capacitor, the other terminal of which isconnected to the 0V line.

For example, the input voltage Vin can comprise a smoothed DC voltageor, particularly in the case of a boost converter used for PFC, arectified AC voltage. By way of further example, for a boost converterto be used for PFC in consumer electronics equipment such as atelevision, the input voltage Vin may be a rectified AC voltage with apeak voltage in a range of the order of 120 to 360V, and the outputvoltage Vout may be of the order of 380 to 400V, for example about 385V.In such an application the converter may be designed for an output powerin a range of, for example, 200 to 700 W, with the converter operated incontinuous current mode (CCM).

As is well known in the art, when the switch 12 is open (the MOSFET isoff or non-conductive), current from the input flows via the boostinductor 10 and the boost diode 14, which is forward biased, to chargethe capacitor 16 and maintain its output voltage Vout while supplyingcurrent to a load (not shown) coupled to the output of the converter.When the switch 12 is closed by the control signal G (the MOSFET isturned on or conductive), while current is flowing in the inductor 10 inthe case of CCM, the inductor current flows via the switch 12, the diode14 is reverse biased, and current to the load is maintained by theoutput capacitor 16.

With such switching of the switch 12 the inductor current is switched bythe MOSFET switch 12 being turned on and off, resulting in undesiredswitching losses. Although a high switching frequency is desirable tofacilitate reducing sizes of the boost inductor 10 and the outputcapacitor 16, such switching losses increase with increasing switchingfrequency and hence impose a practical limit on the switching frequency.

In addition, in the absence of the snubber circuit described below, whenthe MOSFET switch 12 is turned on the boost diode 14 is reverse biased,but remains conductive during its reverse recovery period, resulting inlarge currents flowing during this period, increasing the stressesimposed on the switch 12 and increasing the converter losses.

The snubber circuit of the boost converter of FIG. 1 includes, inaddition to the snubber inductor 20 in series with the boost diode 14, aseries-connected snubber resistor 22 and diode 24 connected in parallelwith the series-connected snubber inductor 20 and boost diode 14, and afurther diode 26 having its anode connected to the 0V line and itscathode connected to the junction between the snubber inductor 20 andthe boost diode 14.

The snubber inductor 20 slows the turn off of the boost diode 14 andhence reduces its reverse recovery losses, and reduces turn-on losses ofthe MOSFET switch 12 by preventing a rapid increase of current. Thevoltage across the MOSFET switch is prevented from ringing, when theswitch is turned off, by the resistor 22 and diode 24 clamping thisvoltage to the output voltage Vout. The further diode 26 conductsnegative current in the snubber inductor when the MOSFET switch 12 isturned on.

This known boost converter has the disadvantage of requiring the diode26 to prevent ringing of the voltage at the junction between the snubberinductor 20 and the boost diode 14. A further disadvantage is that whenthe MOSFET switch 12 is turned off and the diode 26 is forward biased bythe voltage at this junction swinging below 0V, a current through thesnubber inductor 20 circulates via the closed switch 12 and the forwardbiased diode 26, resulting in further losses.

FIG. 2 schematically illustrates a boost converter in accordance with anembodiment of this invention, including the same components 10, 12, 14,and 16 as described above for the boost converter of FIG. 1. Thus in theboost converter of FIG. 2 the inductor 10 and the diode 14 are coupledin series in a series path between the input and output terminals of theconverter, and the MOSFET switch 12 is in a shunt path of the converter.

In addition, the boost converter of FIG. 2 includes a snubber comprisingan inductor 20, resistor 22, and diode 24, which have the samereferences as in FIG. 1, and a capacitance 28. The snubber in the boostconverter of FIG. 2 has no diode 26 as in the snubber of FIG. 1, and itscomponents are connected differently as described further below.

More particularly, in the boost converter of FIG. 2 the inductor 20 isconnected in series with the boost diode 14, in this case between thecathode of the diode 14 and the output terminal for the output voltageVout of the converter. The inductor 20 typically has an inductance muchless than that of the boost inductor 10. For ease of reference,junctions at the anode and cathode of the boost diode 14 of theconverter of FIG. 2 are referenced A and C respectively, and the outputterminal for the voltage Vout is referred to as the junction Vout.

The resistor 22 and diode 24 are connected in series between thejunctions C and Vout, with the diode 24 poled for conducting a currentIr as shown through the resistor 22 in a direction from the junction Ctowards the junction Vout. The series order of the resistor 22 and thediode 24 can optionally be reversed from that shown. Thus either thediode 24 can have its cathode coupled to the junction Vout and its anodecoupled via the resistor 22 to the junction C as shown, or the diode canhave its anode coupled to the junction C and its cathode coupled via theresistor 22 to the junction Vout. In either case the series-connectedresistor 22 and diode 24 are connected in parallel with the inductor 20,not in parallel with the series-connected inductor 20 and diode 14 as inthe converter of FIG. 1.

The capacitance 28 is connected between the junctions A and C, and hencein parallel with the boost diode 14. Depending upon particularcharacteristics of the boost converter, including for example itsswitching frequency and output voltage, the capacitance 28 can beconstituted partly or entirely by parasitic capacitance of the boostdiode 14.

FIG. 3 shows a modified form of the boost converter of FIG. 2, in whichthe series order of the boost diode 14 and the inductor 20 is changed.Thus in the converter of FIG. 3 one terminal of the inductor 20 isconnected to the junction between the drain of the MOSFET switch 12 andthe inductor 10, and the other terminal of the inductor 20 is connectedto the anode of the boost diode 14. The cathode of the boost diode 14 isconnected to the output terminal for the output voltage Vout. As in theconverter of FIG. 2, in the converter of FIG. 3 the series-connectedresistor 22 and diode 24 (in either order) are in parallel with theinductor 20, and the capacitance 28 is in parallel with the boost diode14.

Operation of the converter of FIG. 3 is similar to operation of theconverter of FIG. 2, which is described below with additional referenceto FIGS. 4 to 6, which illustrate waveforms of voltages and currentsthat can occur in operation of the converter. These waveforms aresimplified in that the effects of parasitics are not all shown.

More particularly, each of FIGS. 4 to 6 illustrates voltage waveforms Aand C, in volts (V), at the junctions A and C respectively in FIG. 2,and current waveforms Iq, Id, and Ir, in amps (A), for a current Iq inthe switch 12 (drain-source current of the MOSFET constituting theswitch 12), a current Id in the boost diode 14, and the current Ir inthe resistor 22, as shown by arrows in FIG. 2. FIG. 4 illustrates thewaveforms for a complete switching cycle, and FIGS. 5 and 6 illustratethe waveforms on expanded time scales around the turn-on and turn-offtimes, respectively, of the switch 12. For example, the period of oneswitching cycle from a time t0 to a time t10 in FIG. 4 can be 10 μs, theperiod from the time t0 to a time t3 in FIG. 5 can be of the order ofabout 80 ns, and the period from a time t5 to a time t8 in FIG. 6 can beof the order of about 50 ns.

These waveforms are described for a boost converter having the followingcomponent values and characteristics, which are given here by way ofexample to assist in providing a full understanding; the invention isnot limited in any way to any of these values or characteristics:

Output voltage Vout 385 V Inductor 20 5 μH Switching frequency 100 kHzResistor 22 25 Ω Boost inductor 10 800 μH Capacitance 28 300 pF Outputcapacitor 16 50 μF Output power 400 W

In other embodiments of the invention, all of these values may becompletely different. As just one example, the capacitance 28 can beincreased to several nF with a less hard drive of the MOSFET switch 12,or it can potentially be reduced to the parasitic capacitance of theboost diode 14 for a boost converter with a low output voltage.

Referring particularly to FIGS. 4 and 5, immediately before a time t0 atwhich the control signal G goes high to turn on the MOSFET constitutingthe switch 12, the diode 14 is forward biased to conduct the current Idfrom the input Vin to the output junction Vout via the inductors 10 and20, the currents Iq and Ir are substantially zero, and the junctions Aand C are at substantially the output voltage Vout (the junction Aactually being more positive than the junction C by the forward voltageof the diode 14 at the prevailing current Id).

Starting at the time t0 when the control signal G (not shown) goes high,and until a time t1 very soon afterwards as shown in FIG. 5, the MOSFETturns on (the switch 12 is closed) so that the voltage at the junction Afalls rapidly to substantially zero. Because of the inductor 20 inseries with the diode 14, during the short interval t0-t1 the current Idin the diode 14 and inductor 20 changes very little, the diode 14remains forward biased, and the voltage at the junction C also fallssubstantially to zero at the time t1.

Consequently, as shown in FIG. 5, in the interval t0-t1 the MOSFETswitch 12 is turned on with very little current Iq flowing, and henceunder almost zero current switching (ZCS) conditions with relativelylittle switching loss. At the time t1 the MOSFET switch 12 is fullyturned on and the output voltage Vout appears across the inductor 20.Accordingly the current Id in the forward biased diode 14 and theinductor 20 ramps down, linearly from the time t1, to reach zero at atime t2 somewhat after the time t1 as shown in FIG. 5.

At the time t2 when the current Id reaches zero, the diode 14 becomesreverse biased and the voltage at the junction C rises fromsubstantially zero in a resonant fashion, as best shown by a curve 50 inFIG. 5, due to the capacitance 28 being charged via the inductor 20. Theresonance causes the voltage at the junction C to overshoot the outputvoltage Vout at a time t3, following which the diode 24 becomes forwardbiased and the current Ir rises from substantially zero as best shown bya curve 54 in FIG. 5, energy stored in the inductor 20 being dissipatedin the resistor 22.

As shown in FIGS. 4 and 5 by a curve 52, from the time t0 until the timet2 the current Iq rises in an inverse manner to the fall of the currentId during this period, and from the time t2 until the time t3 thecurrent Iq continues to rise with current flowing via the inductor 20and the capacitance 28 as the voltage at the junction C rises resonantlyas described above. When the diode 24 becomes forward biased starting atthe time t3, the current Iq falls to a steady state value correspondingto its value at the time t2 and the value of the current Id at the timet0. During the remainder of the on period of the MOSFET switch 12, untilthe time t5 as best shown in FIG. 4 by a line 56, the current Iq in theMOSFET switch 12 ramps up from this steady state value to a value Ioff,due to the input voltage Vin applied to the boost inductor 10 by theclosed switch 12.

Referring particularly to FIGS. 4 and 6, immediately before the time t5at which the control signal G goes low to turn off the MOSFET switch 12,the junction A is at 0V and the junction C is at substantially theoutput voltage Vout, the capacitance 28 being charged to the outputvoltage Vout and the diode 14 being reverse biased, so that the currentsId and Ir are substantially zero. The MOSFET switch 12 is on, with itscurrent Iq, conducted via the boost inductor 10, having the value Ioffas shown in FIGS. 4 and 6.

The MOSFET switch 12 is turned off (the switch 12 is opened) during aninterval from the time t5, when the control signal G (not shown) goeslow, until a time t6 at which the MOSFET is fully turned off. Duringthis interval t5-t6 the current Iq of the MOSFET switch 12 falls fromits value Ioff to substantially zero. As the current in the inductors 10and 20 can not change instantaneously, the current in the inductor 10flows via the capacitor 28, the resistor 22, and the diode 24 to theoutput junction Vout, with the voltage at the junction A rising rapidlyto a value Vr=R.Ioff where R is the resistance of the resistor 22. Thevoltage at the junction C is increased correspondingly to a valueVr+Vout, thereby forward biasing the diode 24, and as shown by a line 64the current Ir in the resistor 22 and the diode 24 increases tosubstantially the value Ioff at the time t6.

From the time t6 until a time t7, the capacitance 28 is dischargedsubstantially linearly by the relatively constant current Ir flowing viathe inductor 10, capacitance 28, resistor 22, and forward biased diode24, so that the voltage at the junction A rises substantially linearlyas best shown by a line 60 in FIG. 6. At the time t7 this voltage at thejunction A rises above the voltage at the junction C and forward biasesthe diode 14, which accordingly starts to conduct, its current Idrising, as best shown by a line 62 in FIG. 6, from the time t7 until atime t8 at which the diode 14 conducts all of the current flowing viathe inductor 10.

Following the time t8, as shown in FIG. 4 the voltages at the junctionsA and C fall to substantially the output voltage Vout, the current Irfalls to substantially zero, and the current Id flowing through theinductor 10, diode 14, and (when the current Ir has fallen tosubstantially zero) the inductor 20 ramps down, as shown by a line 66 inFIG. 4, until the time t10 at which the switching cycle repeats. At thetime t10 the current Id reaches substantially the same value as at thetime t0.

The resistance R of the resistor 22 and the magnitude of the capacitance28 are desirably chosen so that the voltage Vr which is attained by thejunction A while the MOSFET switch 12 is turning off is a small fractionof the output voltage Vout; for example as illustrated in FIG. 6 it maybe of the order of 60V or less for an output voltage Vout of the orderof 385V. Consequently, switching losses on turning off the MOSFET switch12 are greatly reduced. For example, turn-off switching losses for theconverter of FIG. 2 may be of the order of 15% or less of the switchinglosses for the same converter without a snubber.

In addition, by choosing the inductance of the inductor 20 to besufficient that the interval t0-t2 is substantially larger than theinterval t0-t1 for turn-on of the MOSFET switch 12, the switching losson turn-on of the MOSFET switch 12 is reduced as described above, forexample to 20% or less of what it would be for the same converterwithout a snubber. Further, because the forward bias of the diode 14 inthe converter of FIG. 2 is maintained until after the MOSFET switch hasbeen fully turned on, the problem of diode reverse recovery is avoided.

Thus while the converter of FIG. 2 still has some losses, these aregreatly reduced in comparison to the losses of a converter without asnubber. Power dissipation in the resistor 24 can for example be of theorder of 1% of the output power of the converter. At the same time, thediode reverse recovery problem is avoided, so that the converter of FIG.2 does not require the use of very fast or very expensive diodes. Theseadvantages of the converter of FIG. 2 are achieved without requiring anadditional switch and its drive circuitry, and without the relativecomplexity and related costs, of soft switching boost converters asdiscussed above. They are also achieved without requiring the furtherdiode 26 as in the converter of FIG. 1, and without any consequentcirculating current through such a diode.

Although the above description relates to a boost converter, similarissues of switching losses and diode reverse recovery arise in otherpower converters, including for example a buck converter, and can beaddressed in accordance with embodiments of the invention in a similarmanner to that described above. For example, FIG. 7 illustrates a buckconverter in accordance with another embodiment of the invention.

Referring to FIG. 7, the buck converter shown therein comprises a MOSFETswitch 70, controlled by a control signal G′ supplied to its gate,coupled in series with an output inductor 74 between a terminal for apositive input voltage Vin and a terminal for a positive output voltageVout which is less than Vin. The buck converter also includes a diode 72having its anode connected to a 0V line and its cathode coupled to apoint between the MOSFET switch 70 and the output inductor 74, and anoutput capacitor 76 coupled between the positive output voltage terminaland the 0V line. Thus in the buck converter of FIG. 7 the MOSFET switch70 and the inductor 74 are coupled in series in a series path betweenthe input and output terminals of the converter. The diode 72 isconnected in a shunt path of the converter.

The buck converter of FIG. 7 also includes a snubber comprising aninductor 80, in series between the MOSFET switch 70 and the outputinductor 74; a series-connected resistor 82 and diode 84, in parallelwith the inductor 80 with the diode 84 poled for conduction in the samedirection as the body diode of the MOSFET switch 70; and a capacitance86 in parallel with the diode 72. The inductor 80 typically has a muchsmaller inductance than the output inductor 74. With a relatively loweroutput voltage, the capacitance 86 may typically be larger than thecapacitance 28 in the boost converter of FIG. 2, and the resistance ofthe resistor 82 may typically be smaller than that of the resistor 22 ofthe boost converter of FIG. 2.

FIG. 7 also shows a junction A′ of the source of the MOSFET switch 70with the inductor 80, and a junction C′ of the cathode of the diode 72with the inductors 80 and 74, which are referred to below. The buckconverter of FIG. 7 operates in a manner that can be correlated to theoperation of the boost converter of FIG. 2 as described in detail above,and is summarized below.

Immediately before the MOSFET switch 70 is turned on, the junctions A′and C′ are at substantially 0V, and there is substantially zero currentthrough the MOSFET switch 70 and the resistor 82. The diode 72 isforward biased and conducting current via the inductor 74 to thecapacitor 76 and the output. Under the control of the control signal G′,the MOSFET switch 70 is turned on rapidly and the voltage at thejunction A′ rises quickly to the input voltage Vin, with the diode 72still forward biased and its current ramping down relatively slowly tozero, current through the MOSFET switch 70 increasing conversely. Thevoltage at the junction C′ then rises resonantly due to the capacitance86 and inductance 80, with current through the MOSFET switch 70 rising,until the diode 84 becomes forward biased. Then energy of the inductor80 is dissipated in the resistor 82. The current through the MOSFETswitch 70 accordingly falls to a steady state, from which it ramps upslowly until the MOSFET switch is turned off. While the current throughthe MOSFET switch 70 is ramping up, the voltage at the junction C′ fallsto the input voltage Vin.

When the control signal G′ turns off the MOSFET switch 70, currentthrough the inductor 80 flows via the diode 84 and resistor 82 insteadof through the switch. Consequently the switch current falls rapidly tozero and the voltage at the junction A′ falls rapidly by the product ofthis current and the resistance of the resistor 82. The voltages at thejunctions A′ and C′ then fall relatively slowly, until the voltage atthe junction A′ has become negative and the voltage at the junction C′crosses zero and forward biases the diode 72. Current then flows via thediode 72 and the output inductor 74, ramping down slowly until theMOSFET switch 70 is next turned on, with the voltages at the junctionsA′ and C′ returning to substantially 0V and the current through theresistor 82 falling to zero.

As the MOSFET switch 70 is directly in series with the inductor 80 withits parallel series-connected resistor 82 and diode 84, it will beappreciated that the positions of these can be exchanged; thus theinductor 80 with its parallel series-connected resistor 82 and diode 84can instead be connected between the terminal for the input voltage Vinand the MOSFET switch 70. In either case the inductor 80 is in serieswith the MOSFET switch 70, in the series path between the input andoutput terminals of the converter.

Another alternative circuit arrangement of the buck converter isillustrated in FIG. 8, in which, instead of being connected in serieswith the MOSFET switch 70 as in FIG. 7, the inductor 80 and its parallelseries-connected resistor 82 and diode 84 are connected in series withthe diode 72 and its parallel capacitance 86, i.e. in the shunt path ofthe converter. Thus as shown in FIG. 8, the inductor 80, and likewisethe series-connected resistor 82 and diode 84, are connected between thecathode of the diode 72 and the junction of the MOSFET switch 70 withthe output inductor 74.

Alternatively, the cathode of the diode 72 can be connected to thejunction of the MOSFET switch 70 and the output inductor 74, and theinductor 80 can be connected between the anode of the diode 72 and the0V line, with the capacitance 86 in parallel with the diode 72 and theseries-connected resistor 82 and diode 84 in parallel with the inductor80.

It can be appreciated that in each of the power converters of FIGS. 2,3, 7, and 8, and the alternatives discussed above, the snubber inductor20 or 80 is arranged so that it is in a series path which includes boththe converter switch 12 or 70 and the converter diode 14 or 72. Theinductor 20 or 80 prevents a very rapid change of current through theconverter diode 14 or 72 when the MOSFET switch 12 or 70 is turned on,so that the diode remains forward biased until after the MOSFET switchis fully turned on. In addition, in each of these power converters andthe alternatives discussed above, the series-connected resistor 22 or 82and diode 24 or 84 are connected in parallel with the snubber inductor20 or 80, and the capacitance 28 or 86, to the extent that it is notprovided by the capacitance of the converter diode 14 or 72, is added inparallel with this diode. The invention also applies to other circuitarrangements, in buck or boost converters, other power converters, orother circuits such as may be used for motor control, relay control, andso on, that have similar relevant characteristics.

From this, it can be seen for example that other embodiments of theinvention can apply to a boost converter as shown in FIG. 9.

Referring to FIG. 9, in which the same components as in the boostconverter of FIGS. 2 and 3 are used and have the same references, theinductor 20, and the series-connected resistor 22 and diode 24 inparallel with the inductor 20, are moved to a different position in thepath that includes the converter MOSFET switch 12 and the boost diode14, in this case in the shunt path of the converter, between the drainof the MOSFET switch 12 and the junction of the inductor 10 with thediode 14. The capacitance 28 is still connected in parallel with thediode 14.

It can be seen that the boost converter of FIG. 9 can be furthermodified by interchanging the positions, in the shunt path of theconverter, of the MOSFET switch 12 and the inductor 20, with theresistor 22 and diode 24 remaining in parallel with the inductor 20,and/or by interchanging the positions of the series-connected resistor22 and diode 24.

It can further be appreciated that the snubber inductor 20 or 80, withthe series-connected resistor 22 or 82 and diode 24 or 84 in parallelwith the inductor 20 or 80, can instead be moved to a position in the 0Vline, between the MOSFET switch 12 and the output capacitor 16 in thecase of a boost converter, and between the 0V input terminal and theconverter diode 72 in the case of a buck converter.

FIGS. 10 to 12 illustrate modifications of the boost converter of FIG. 2in accordance with further embodiments of the invention. Similarmodifications can be applied to the converters of FIG. 3 and FIGS. 7 to9.

In FIG. 10, the boost converter of FIG. 2 is modified by providing anadditional capacitor 90 in parallel with the resistor 22. The additionof the capacitor 90 has the advantages of reducing peak voltage across,and peak current through, the resistor 22. Current through the resistor22 in this case flows for a longer time, so that there is no change inpower dissipated by the resistor 22. This capacitor 90 in parallel withthe resistor 22 is also shown in dashed lines in each of FIGS. 7 to 9 toindicate that it may optionally be provided in the power converters ofthese figures.

In FIG. 11, the boost converter of FIG. 10 is further modified byincorporating the capacitance of the capacitor 90 into the capacitor 28,which accordingly is connected between the anode of the diode 14 and thejunction between the resistor 22 and the diode 24. The capacitor 28 isthus connected in parallel with the diode 14 and the resistor 22 inseries. In FIG. 12, the series order of the resistor 22 and the diode 24is reversed. The capacitor 28 is again connected between the anode ofthe diode 14 and the junction between the resistor 22 and the diode 24.Thus in this case the capacitor 28 is connected in parallel with thediode 14 and the diode 24 in series.

It can be appreciated that, in any instance where a terminal of thecapacitor 28 or 90 is connected to a point at a substantially DC level,it can be connected instead to any other point at a substantially DClevel. For example, in the boost converter of FIG. 3, instead of beingconnected between the anode of the diode 14 and the cathode of the diode14 which is at the substantially DC output voltage Vout, the capacitor28 can be coupled between the anode of the diode 14 and the 0V line.Applying this principle and the modification of FIG. 11 or FIG. 12 tothe converter of FIG. 3, the capacitor 28 can instead be connectedbetween the junction of the resistor 22 and the diode 24, in series ineither the order shown in FIG. 3 or the reverse order, and either thecathode of the diode 14 at the substantially DC output voltage Vout orthe 0V line, or the terminal for the voltage Vin if this is a DC inputvoltage.

Thus, although particular embodiments of the invention are describedabove by way of example, it can be appreciated that numerousmodifications, variations, and adaptations may be made without departingfrom the scope of the invention as defined in the claims.

For example, one aspect of this invention might provide a powerconverter comprising two input terminals, two output terminals, anoutput capacitor coupled between the two output terminals, a firstinductor in a series path between the input and output terminals, aswitch controlled by a control signal, and a diode, the converter havinga configuration for producing an output voltage at the output terminalsfrom an input voltage supplied to the input terminals, the converterfurther comprising a second inductor, and a series-connected resistorand second diode in parallel with the second inductor, in a path inseries with the switch and the first diode.

The first inductor and the switch can be coupled in series between thetwo input terminals, with the first diode in said series path betweenthe input and output terminals, to provide a boost configuration of thepower converter. In this case the second inductor can be in series withthe first diode in said series path between the input and outputterminals, or it can be in series with the switch in a shunt path of theconverter.

Alternatively, the first inductor and the first diode can be coupled inseries between the two output terminals, with the switch in said seriespath between the input and output terminals, to provide a buckconfiguration of the power converter. In this case the second inductorcan be in series with the switch in said series path between the inputand output terminals, or it can be in series with the diode in a shuntpath of the converter.

A boost converter provided by another aspect of the invention comprisestwo input terminals, a first inductor and a controlled switch coupled inseries between the two input terminals, a first diode and an outputcapacitor coupled in series across the switch, and a second inductor anda series-connected resistor and second diode in parallel with the secondinductor, the second inductor and series-connected resistor and seconddiode in parallel therewith being in series with the first diode.

A boost converter provided by a further aspect of the inventioncomprises two input terminals, a first inductor and a controlled switchcoupled in series between the two input terminals, a first diode and anoutput capacitor coupled in series across the switch, and a secondinductor and a series-connected resistor and second diode in parallelwith the second inductor, the second inductor and series-connectedresistor and second diode in parallel therewith being in series with theswitch.

A buck converter provided by another aspect of the invention comprisestwo input terminals, a controlled switch and a first diode coupled inseries between the two input terminals, a first inductor and an outputcapacitor coupled in series across the first diode, and a secondinductor and a series-connected resistor and second diode in parallelwith the second inductor, the second inductor and series-connectedresistor and second diode in parallel therewith being in series with theswitch.

A buck converter provided by a further aspect of the invention comprisestwo input terminals, a controlled switch and a first diode coupled inseries between the two input terminals, a first inductor and an outputcapacitor coupled in series across the first diode, and a secondinductor and a series-connected resistor and second diode in parallelwith the second inductor, the second inductor and series-connectedresistor and second diode in parallel therewith being in series with thediode.

Operation of each of the above converters benefits from a capacitance inparallel with the first diode. A parasitic capacitance of the diode canconceivably constitute all of this capacitance in some cases, butpreferably a capacitor is connected in parallel with the first diode.Another capacitor can also be coupled in parallel with the resistor, oralternatively the capacitor can be connected in parallel with the firstdiode in series with the resistor or the second diode.

Some embodiments of the invention also extend to a circuit arrangementcomprising: a first inductor through which a current flows in operationof the circuit arrangement; a switch arranged to be opened and closedunder the control of a control signal, the switch being arranged forconducting current of the first inductor when the switch is closed; anda first diode arranged to be forward biased for conducting current ofthe inductor when the switch is open and for being reverse biased whenthe switch is closed; wherein the circuit arrangement further comprises:a second inductor, having an inductance much less than an inductance ofthe first inductor; and a resistor and a second diode connected inseries with the resistor, the series-connected resistor and second diodebeing connected in parallel with the second inductor; the secondinductor with the series-connected resistor and second diode in paralleltherewith being in a path in series with the switch and the first diode.

The circuit arrangement may also include a capacitor connected inparallel with the first diode. The circuit arrangement can form a boostconverter having input and output terminals, the first inductor couplingthe input terminals to the switch, and the first diode coupling ajunction between the first inductor and the switch to the outputterminals. Alternatively, the circuit arrangement can form a buckconverter having input and output terminals, the first inductor couplingthe output terminals to the first diode, and the switch coupling ajunction between the first inductor and the first diode to the inputterminals.

1. A power converter comprising: three terminals; a switch coupled to beturned on and off in response to a control signal; a first diode havingfirst and second terminals; a first inductor coupling a first junctionin a path between the switch and the first diode to a third terminal ofthe three terminals; and a snubber circuit coupled to reduce switchinglosses in the power converter, the snubber circuit comprising: a secondinductor coupled to the first terminal of the first diode, a firstterminal of the three terminals being coupled to a second terminal ofthe three terminals through the switch, the second inductor, and thefirst diode; a second diode coupled to a first end of the secondinductor; a resistor coupled to a second end of the second inductor andto the second diode; and a capacitor coupled between the second terminalof the first diode and a second junction between the second diode andthe resistor, wherein the switch being turned on causes the secondinductor, the second diode, the resistor and the capacitor of thesnubber circuit as coupled to maintain a forward bias of the first diodeas the switch is turned on, and wherein the switch being turned offcauses the second inductor, the second diode, the resistor and thecapacitor of the snubber circuit as coupled to attain a voltage at theswitch that is a fraction of a voltage at the second terminal of thethree terminals as the switch is turned off.
 2. A power converter asclaimed in claim 1, wherein the first terminal comprises a voltagereference terminal, the second terminal comprises an output terminal,and the third terminal comprises an input terminal, to provide a boostconfiguration of the power converter.
 3. A power converter as claimed inclaim 2, wherein the second inductor and the first diode are coupledbetween the first junction and the output terminal.
 4. A power converteras claimed in claim 2, wherein the second inductor and the switch arecoupled between the voltage reference terminal and the first junction.5. A power converter as claimed in claim 2, further comprising: anoutput capacitor coupled across the output terminal and the voltagereference terminal.
 6. A power converter as claimed in claim 1, thefirst terminal comprises an input terminal, the second terminalcomprises a voltage reference terminal, and the third terminal comprisesan output terminal, to provide a buck configuration of the powerconverter.
 7. A power converter as claimed in claim 6, wherein thesecond inductor and the switch are coupled between the input terminaland the first junction.
 8. A power converter as claimed in claim 6,wherein the second inductor and the first diode are coupled between thefirst junction and the voltage reference terminal.
 9. A power converteras claimed in claim 6, further comprising: an output capacitor coupledacross the output terminal and the voltage reference terminal.
 10. Aboost converter comprising: a voltage reference terminal; an inputterminal to receive an input voltage relative to the voltage referenceterminal; an output terminal to provide an output voltage relative tothe voltage reference terminal; a switch coupled to be turned on and offin response to a control signal; a first diode having first and secondterminals; a first inductor coupling a first junction in a path betweenthe switch and the first diode to the input terminal; and a snubbercircuit coupled to reduce switching losses in the boost converter, thesnubber circuit comprising: a second inductor coupled to the firstterminal of the first diode, the output terminal being coupled to thevoltage reference terminal through the switch, the second inductor, andthe first diode; a second diode coupled to a first end of the secondinductor; a resistor coupled to a second end of the second inductor andto the second diode; and a capacitor coupled between the second terminalof the first diode and a second junction between the second diode andthe resistor, wherein the switch being turned on causes the secondinductor, the second diode, the resistor and the capacitor of thesnubber circuit as coupled to maintain a forward bias of the first diodeas the switch is turned on, and wherein the switch being turned offcauses the second inductor, the second diode, the resistor and thecapacitor of the snubber circuit as coupled to attain a voltage at theswitch that is a fraction of a voltage at the output terminal as theswitch is turned off.
 11. A boost converter as claimed in claim 10,wherein the second inductor and the first diode are coupled between theoutput terminal and the first junction.
 12. A boost converter as claimedin claim 10, wherein the second inductor and the switch are coupledbetween the voltage reference terminal and the first junction.
 13. Aboost converter as claimed in claim 10, further comprising: an outputcapacitor coupled across the output terminal and the voltage referenceterminal.
 14. A buck converter comprising: a voltage reference terminal;an input terminal to receive an input voltage relative to the voltagereference terminal; an output terminal to provide an output voltagerelative to the voltage reference terminal; a first diode; a controlledswitch; a first inductor coupling a first junction in a path between thecontrolled switch and the first diode to the output terminal; and asnubber circuit coupled to reduce switching losses in the buckconverter, the snubber circuit comprising: a second inductor, the inputterminal being coupled to the voltage reference terminal through thecontrolled switch, the second inductor, and the first diode; a seconddiode coupled to a first end of the second inductor; a resistor coupledto a second end of the second inductor and to the second diode; and acapacitor coupled across the first diode and to the second diode and theresistor, wherein the controlled switch being turned on causes thesecond inductor, the second diode, the resistor and the capacitor of thesnubber circuit as coupled to maintain a forward bias of the first diodeas the controlled switch is turned on, and wherein the controlled switchbeing turned off causes the second inductor, the second diode, theresistor and the capacitor of the snubber circuit as coupled causevoltages at the first and second ends of the second inductor to fallslowly until the first diode is forward biased as the controlled switchis turned off.
 15. A buck converter as claimed in claim 14, wherein theswitch and the second inductor are coupled between the input terminaland the first junction.
 16. A buck converter as claimed in claim 14,wherein the first diode and the second inductor are coupled between thefirst junction and the voltage reference terminal.
 17. A buck converteras claimed in claim 14, further comprising: an output capacitor coupledacross the output terminal and the voltage reference terminal.